The present invention relates to position determination and radio navigation. In particular, the present invention relates to methods and apparatus for determining the relative bearing and range between two mobile objects (e.g. aircraft and aircraft carrier) or between a mobile object (e.g. aircraft) and a stationary object (i.e. TACAN ground station) with a low probability of detection of radio signals by an unfriendly opponent.
Mobile objects, such as aircraft flying from an aircraft carrier sometimes require methods for determining range and bearing to a reference object which may be mobile or stationary, such as the ship. This is especially important when the reference object moves, such as when a carrier conducts evasive maneuvers after launching aircraft for an attack. In one system for determining range and bearing information, known as TACAN, a shipboard or ground installation, or another aircraft transmits high power pulse pair radio signals from which an aircraft can determine relative range and bearing. The ship or ground installation has a rotating antenna which amplitude modulates these pulse pairs as it sweeps through a 360 degree revolution. The power radiated along any particular direction varies with time, having a period equal to antenna RPM. To an aircraft, the signal envelope of the pulse pairs appears to be amplitude modulated with the peak occurring each time the antenna points directly at the aircraft. As the antenna beam sweeps through a North reference bearing, the ship transmits a series of closely spaced pulses which are referred to as a North burst signal. When an aircraft is due east of the carrier, the North burst will coincide with the envelope peak. When an aircraft is positioned along any other bearing, the north burst and the maximum amplitude signal will be separated in time, and the aircraft can determine bearing from measuring elapsed time between the North burst and the maximum signal amplitude. By thus detecting this elapsed time, an aircraft can passively determine bearing to the beacon (e.g. ship, ground installation, or other aircraft).
Aircraft must operate in an active mode to determine range. An aircraft can transmit an interrogation pulse pair. When the ship receives the aircraft's interrogation pulses, the ship delays a fixed time interval and replies by transmitting a pulse pair. The aircraft must correlate several interrogations to determine the reply pulse from all other pulse pairs transmitted by the beacon. The aircraft then computes the round trip radio propagation time delay by measuring the total delay between the aircraft's transmission pulses and the reception of the ship's reply, and then subtracting out the fixed delay. The range to the beacon is simply the one way propagation delay time multiplied by the radio signal propagation velocity.
Systems which, like TACAN, use a high power radio signal create risk to the beacon installation itself (e.g. aircraft carrier). Hostile forces can use the RF signal for locating the beacon and launching an attack. Moreover, weapons may use the beacon for targeting, via radio seeking missiles. Ships turn off their beacons when they suspect an enemy may be nearby. The aircraft are unable to obtain range and bearing during the off time; therefore, the TACAN is useless. An alternate relative navigation system is needed, especially with the proliferation of intelligent missiles which can home in on radio transmitters.
There are non-threat related instances where low probability of intercept (LPI) relative navigation is desirable. For example, LPI signals sometimes cause less interference with other radio transmissions (fratricide).
It is sometimes also desirable, to retain the normal, high power capability for cases where no enemy operations are present. This can allow compatibility with multiple systems. Commonality of certain functions, such as operator interface, simplifies operation and reduces cost and training impact.
A simple-to-operate, low-cost method for providing existing aircraft with a relative navigation system having a low probability of intercept (LPI) is thus highly desirable. Because of the high cost of retrofitting many aircraft and the interruption of operations which accompany such replacement, an add-on unit (modification to existing equipment) is especially desirable from a cost, schedule, and risk point of view. Retaining compatibility with existing equipment constrains possible solutions.